JOSE R. Barnes
Internet-Draft BBN Technologies
Intended status: Informational October 22, 2012
Expires: April 25, 2013
Use Cases and Requirements for JSON Object Signing and Encryption (JOSE)
draft-barnes-jose-use-cases-01.txt
Abstract
Many Internet applications have a need for object-based security
mechanisms in addition to security mechanisms at the network layer or
transport layer. In the past, the Cryptographic Message Syntax has
provided a binary secure object format based on ASN.1. Over time,
the use of binary object encodings such as ASN.1 has been overtaken
by text-based encodings, for example JavaScript Object Notation.
This document defines a set of use cases and requirements for a
secure object format encoded using JavaScript Object Notation, drawn
from a variety of application security mechanisms currently in
development.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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Internet-Draft JOSE Use Cases October 20121. Introduction
Internet applications rest on the layered architecture of the
Internet, and take advantage of security mechanisms at all layers.
Many applications rely primarily on channel-based security
technologies, which create a secure channel at the IP layer or
transport layer over which application data can flow
[RFC4301][RFC5246]. These mechanisms, however, cannot provide end-
to-end security in some cases. For example, in protocols with
application-layer intermediaries, channel-based security protocols
would protect messages from attackers between intermediaries, but not
from the intermediaries themselves. These cases require object-based
security technologies, which embed application data within a secure
object that can be safely handled by untrusted entities.
The most well-known example of such a protocol today is the use of
Secure/Multipurpose Internet Mail Extensions (S/MIME) protections
within the email system [RFC5751][RFC5322]. An email message
typically passes through a series of intermediate Mail Transfer
Agents (MTAs) en route to its destination. While these MTAs often
apply channel-based security protections to their interactions (e.g.,
[RFC3207]), these protections do not prevent the MTAs from
interfering with the message. In order to provide end-to-end
security protections in the presence of untrusted MTAs, mail users
can use S/MIME to embed message bodies in a secure object format that
can provide confidentiality, integrity, and data origin
authentication.
S/MIME is based on the Cryptographic Message Syntax for secure
objects (CMS) [RFC5652]. CMS is defined using Abstract Syntax
Notation 1 (ASN.1) and traditionally encoded using the ASN.1
Distinguished Encoding Rules (DER), which define a binary encoding of
the protected message and associated parameters [ITU.X690.1994]. In
recent years, usage of ASN.1 has decreased (along with other binary
encodings for general objects), while more applications have come to
rely on text-based formats such as the Extensible Markup Language
(XML) or the JavaScript Object Notation (JSON)
[W3C.REC-xml-1998][RFC4627].
Many current applications thus have much more robust support for
processing objects in these text-based formats than ASN.1 objects;
indeed, many lack the ability to process ASN.1 objects at all. To
simplify the addition of object-based security features to these
applications, the IETF JSON Object Signing and Encryption (JOSE)
working group has been chartered to develop a secure object format
based on JSON. While the basic requirements for this object format
are straightforward -- namely, confidentiality and integrity
mechanisms, encoded in JSON -- early discussions in the working group
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indicated that many applications hoping to use the formats define in
JOSE have additional requirements. This document summarizes the use
cases for JOSE envisioned by those applications and the resulting
requirements for security mechanisms and object encoding.
Some systems that use XML have specified the use of XML-based
security mechanisms for object security, namely XML Digital
Signatures and XML Encryption
[W3C.CR-xmldsig-core2-20120124][W3C.CR-xmlenc-core1-20120313]. These
mechanisms are defined for use with several security token systems
(e.g., SAML, WS-Federation, and OpenID connect
[OASIS.saml-core-2.0-os][WS-Federation][OpenID.Messages]) and the CAP
emergency alerting format [CAP]. In practice, however, XML-based
secure object formats introduce similar levels of complexity to
ASN.1, so developers that lack the tools or motivation to handle
ASN.1 aren't able to use XML security either. This situation
motivates the creation of a JSON-based secure object format that is
simple enough to implement and deploy that it can be easily adopted
by developers with minimal effort and tools.
2. Definitions
This document makes extensive use of standard security terminology
[RFC4949]. In addition, because the use cases for JOSE and CMS are
similar, we will sometimes make analogies to some CMS concepts
[RFC5652].
The JOSE working group charter calls for the group to define three
basic JSON object formats:
1. Confidentiality-protected object format
2. Integrity-protected object format
3. A format for expressing public keys
In the below, we will refer to these as the "encrypted object
format", the "signed object format", and the "key format",
respectively. In general, where there is no need to distinguish
between asymmetric and symmetric operations, we will use the terms
"signing", "signature", etc. to denote both true digital signatures
involving asymmtric cryptography as well as message authentication
codes using symmetric keys(MACs).
In the lifespan of a secure object, there are two basic roles, an
entity that creates the object (e.g., encrypting or signing a
payload), and an entity that uses the object (decrypting, verifying).
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We will refer to these roles as "sender" and "recipient",
respectively. Note that while some requirements and use cases may
refer to these as single entities, each object may have multiple
entities in each role. For example, a message may be signed by
multiple senders, or decrypted by multiple recipients.
3. Basic Requirements
Obviously, for the encrypted and signed object formats, the necessary
protections will be created using appropriate cryptographic
mechanisms: symmetric or asymmetric encryption for confidentiality
and MACs or digital signatures for integrity protection. In both
cases, it is necessary for the JOSE format to support both symmetric
and asymmetric operations.
o The JOSE encrypted object format MUST support object encryption in
the case where the sender and receiver share a symmetric key.
o The JOSE encrypted object format MUST support object encryption in
the case where the sender has only a public key for the receiver.
o The JOSE signed object format MUST integrity protection using
Message Authentication Codes (MACs), for the case where the sender
and receiver share a symmetric key.
o The JOSE signed object format MUST integrity protection using
digital signatures, for the case where the receiver has only a
public key for the sender.
The purpose of the key format is to provide the recipient with
sufficient information to use the encoded key to process
cryptographic messages. Thus it is necessary to include additional
parameters along with the bare key.
o The JOSE key format MUST include all algorithm parameters
necesssary to use the encoded key, including an identifier for the
algorithm with which the key is used as well as any additional
parameters required by the algorithm (e.g., elliptic curve
parameters).
4. Use Cases
Based on early discussions of JOSE, several working groups developing
application-layer protocols have expressed a desire to use JOSE in
their designs for end-to-end security features. In this section, we
summarize the use cases proposed by these groups and discuss the
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requirements that they imply for the JOSE object formats.
4.1. Security Tokens and Authorization
Security tokens are a common use case for object-based security, for
example, SAML assertions [OASIS.saml-core-2.0-os]. Security tokens
are used to convey information about a subject entity ("claims" or
"assertions") from an issuer to a recipient. The security features
of a token format enable the recipient to verify that the claims came
from the issuer and, if the object is confidentiality-protected, that
they were not visible to other parties.
Security tokens are used in federation protocols such as SAML 2.0,
WS-Federation, and OpenID Connect
[OASIS.saml-core-2.0-os][WS-Federation][OpenID.Messages], as well as
in resource authorization protocols such as OAuth 2.0 [RFC6749]. In
some cases, security tokens are used for client authentication and
for access control
[I-D.ietf-oauth-jwt-bearer][I-D.ietf-oauth-saml2-bearer].
The OAuth protocol defines a mechanism for distributing and using
authorization tokens using HTTP [RFC6749]. A Client that wishes to
access a protected resource requests authorization from the Resource
Owner. If the Resource Owner allows this access, he directs an
Authorization Server to issue an access token to the Client. When
the Client wishes to access the protected resource, he presents the
token to the relevant Resource Server, which verifies the validity of
the token before providing access to the protected resource.
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+---------------+ +---------------+
| | | |
| Resource |<........>| Authorization |
| Server | | Server |
| | | |
+---------------+ +---------------+
^ |
| |
| |
| |
| |
+------------|--+ +--|------------+
| +----------------+ |
| | | Resource |
| Client | | Owner |
| | | |
+---------------+ +---------------+
Figure 1: The OAuth process
In effect, this process moves the token from the Authorization Server
(as a sender of the object) to the Resource Server (recipient), via
the Client as well as the Resource Owner (the latter because of the
HTTP mechanics underlying the protocol). So again we have a case
where an application object is transported via untrusted
intermediaries.
This application has two essential security requirements: Integrity
and data origin authentication. Integrity protection is required so
that the Resource Owner and the Client cannot modify the permission
encoded in the token. Data origin authentication is required so that
the Resource Server can verify that the token was issued by a trusted
Authorization Server. Confidentiality protection may also be needed,
if the Authorization Server is concerned about the visibility of
permissions information to the Resource Owner or Client. For
example, permissions related to social networking might be considered
private information. Note, however, that OAuth already requires that
the underlying HTTP transactions be protected by TLS, so
confidentiality protection is not strictly necessary for this use
case.
The confidentiality and integrity needs are met by the basic
requirements for signed and encrypted object formats, whether the
signing and encryption are provided using asymmetric or symmetric
cryptography. The choice of which mechanism is applied will depend
on the relationship between the two servers, namely whether they
share a symmetric key or only public keys.
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Authentication requirements will also depend on deployment
characteristics. Where there is a relatively strong binding between
the resource server and the authorization server, it may suffice for
the Authorization Server issuing a token to be identified by the key
used to sign the token. This requires that the token carry either
the public key of the Authorization Server or an identifier for the
public or symmetric key.
There may also be more advanced cases, where the Authorization
Server's key is not known in advance to the Resource Server. This
may happen, for instance, if an entity instantiated a collection of
Authorization Servers (say for load balancing), each of which has an
independent key pair. In these cases, it may be necessary to also
include a certificate or certificate chain for the Authorization
Server, so that the Resource Server can verify that the Authorization
Server is an entity that it trusts.
The HTTP transport for OAuth imposes a particular constraint on the
encoding. In the OAuth protocol, tokens frequently need to be passed
as query parameters in HTTP URIs [RFC2616], after having been
base64url encoded [RFC4648]. While there is no specified limit on
the length of URIs (and thus of query parameters), in practice URIs
of more than around 2,000 characters are rejected by some user
agents. So this use case requires that a JOSE object have
sufficiently small size even after signing, possibly encrypting,
while still being simple to include in an HTTP URI query parameter.
Two related security token systems have similar requirements:
o The JSON Web Token format (JWT) is a security token format based
on JSON and JOSE [I-D.ietf-oauth-json-web-token]. It is used with
both OpenID Connect and OAuth. Because JWTs are often used in
contexts with limited space (e.g., HTTP query parameters), it is a
core requirement for JWTs, and thus JOSE, to have a compact
representation.
o The OpenID Connect protocol is a simple, REST/JSON-based identity
federation protocol layered on OAuth 2.0 [OpenID.Messages]. It
uses the JWT and JOSE formats both to represent security tokens
and to provide security for other protocol messages (signing and
optionally encryption).
4.2. XMPP
The Extensible Messaging and Presence Protocol (XMPP) routes messages
from one end client to another by way of XMPP servers [RFC6120].
There are typically two servers involved in delivering any given
message: The first client (Alice) sends a message for another client
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(B) to her server (A). Server A uses Bob's identity and the DNS to
locate the server for Bob's domain (B), then delivers the message to
that server. Server B then routes the message to Bob.
+-------+ +----------+ +----------+ +-----+
| Alice |-->| Server A |-->| Server B |-->| Bob |
+-------+ +----------+ +----------+ +-----+
Figure 2: Delivering an XMPP message
The untrusted-intermediary problems are especially acute for XMPP
because in many current deployments, the holder of an XMPP domain
outsources the operation of the domain's servers to a different
entity. In this environment, there is a clear risk of exposing the
domain holder's private information to the domain operator. XMPP
already has a defined mechanism for end-to-end security using S/MIME,
but it has failed to gain widespread deployment [RFC3923], in part
because of key management challenges and because of the difficulty of
processing S/MIME objects.
The XMPP working group is in the process of developing a new end-to-
end encryption system with an encoding based on JOSE and a clearer
key management system [I-D.miller-xmpp-e2e]. The process of sending
an encrypted message in this system involves two steps: First, the
sender generates a symmetric Content Encryption Key (CEK), encrypts
the message content, and sends the encrypted message to the desired
set of recipients. Second, each recipient "dials back" to the
sender, providing his public key; the sender then responds with the
relevent CEK, wrapped with the recipient's public key.
+-------+ +----------+ +----------+ +-----+
| Alice |<->| Server A |<->| Server B |<->| Bob |
+-------+ +----------+ +----------+ +-----+
| | | |
|------------Encrypted--message--------->|
| | | |
|<---------------Public-key--------------|
| | | |
|---------------Wrapped CEK------------->|
| | | |
Figure 3: Delivering a secure XMPP message
The main thing that this system requires from the JOSE formats is
confidentiality protection via content encryption, plus an integrity
check via a MAC derived from the same symmetric key. The separation
of the key exchange from the transmission of the encrypted content,
however, requires that the JOSE encrypted object format allow wrapped
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symmetric keys to be carried separately from the encrypted payload.
In addition, the encrypted object will need to have a tag for the key
that was used to encrypt the content, so that the recipient (Bob) can
present the tag to the sender (Alice) when requesting the wrapped
key.
Another important feature of XMPP is that it allows for the
simultaneous delivery of a message to multiple recipients. In the
diagrams above, Server A could deliver the message not only to Server
B (for Bob) but also to Servers C, D, E, etc. for other users. In
such cases, to avoid the multiple "dial back" transactions implied by
the above mechanism, XMPP systems will likely cache public keys for
end recipients, so that wrapped keys can be sent along with content
on future messages. This implies that the JOSE encrypted object
format must support the provision of multiple versions of the same
wrapped CEK (much as a CMS EnvelopedData structure can include
multiple RecipientInfo structures).
In the current draft of the XMPP end-to-end security system, each
party is authenticated by virtue of the other party's trust in the
XMPP message routing system. The sender is authenticated to the
receiver because he can receive messages for the identifier "Alice"
(in particular, the request for wrapped keys), and can originate
messages for that identifier (the wrapped key). Likewise, the
receiver is authenticated to the sender because he received the
original encrypted message and originated the request for wrapped
key. So the authentication here requires not only that XMPP routing
be done properly, but also that TLS be used on every hop. Moreover,
it requires that the TLS channels have strong authentication, since a
man in the middle on any of the three hops can masquerade as Bob and
obtain the key material for an encrypted message.
Because this authentication is quite weak (depending on the use of
transport-layer security on three hops) and unverifiable by the
endpoints, it is possible that the XMPP working group will integrate
some sort of credentials for end recipients, in which case there
would need to be a way to associate these credentials with JOSE
objects.
Finally, it's worth noting that XMPP is based on XML, not JSON. So
by using JOSE, XMPP will be carrying JSON objects within XML. It is
thus a desirable property for JOSE objects to be encoded in such a
way as to be safe for inclusion in XML. Otherwise, an explicit CDATA
indication must be given to the parser to indicate that it is not to
be parsed as XML. One way to meet this requirement would be to apply
base64url encoding, but for XMPP messages of medium-to-large size,
this could impose a fair degree of overhead.
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Internet-Draft JOSE Use Cases October 20124.3. ALTO
Application-Layer Traffic Optimization (ALTO) is a system for
distributing network topology information to end devices, so that
those devices can modify their behavior to have a lower impact on the
network [I-D.ietf-alto-reqs]. The ALTO protocol distributes topology
information in the form of JSON objects carried in HTTP
[RFC2616][I-D.ietf-alto-protocol]. The basic version of ALTO is
simply a client-server protocol, so simple use of HTTPS suffices for
this case [RFC2818]. However, there is beginning to be some
discussion of use cases for ALTO in which these JSON objects will be
distributed through a collection of intermediate servers before
reaching the client, while still preserving the ability of the client
to authenticate the original source of the object. Even the base
ALTO protocol notes that "ALTO clients obtaining ALTO information
must be able to validate the received ALTO information to ensure that
it was generated by an appropriate ALTO server."
In this case, the security requirements are straightforward. JOSE
objects carrying ALTO payloads will need to bear digital signatures
from the originating servers, which will be bound to certificates
attesting to the identities of the servers. There is no requirement
for confidentiality in this case, since ALTO information is generally
public.
The more interesting questions are encoding questions. ALTO objects
are likely to be much larger than payloads in the two cases above,
with sizes of up to several megabytes. Processing of such large
objects can be done more quickly if it can be done in a single pass,
which may be possible if JOSE objects require specific orderings of
fields within the JSON structure.
In addition, because ALTO objects are also encoded as JSON, they are
already safe for inclusion in a JOSE object. Signed JOSE objects
will likely carry the signed data in a string alongside the
signature. JSON objects have the property that they can be safely
encoded in JSON strings. All they require is that unnecessary white
space be removed, a much simpler transformation than, say base64url
encoding. This raises the question of whether it might be possible
to optimize the JOSE encoding for certain "JSON-safe" cases.
4.4. Emergency Alerting
Emergency alerting is an emerging use case for IP networks
[I-D.ietf-atoca-requirements]. Alerting systems allow authorities to
warn users of impending danger by sending alert messages to connected
devices. For example, in the event of hurricane or tornado, alerts
might be sent to all devices in the path of the storm.
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The most critical security requirement for alerting systems is that
it must not be possible for an attacker to send false alerts to
devices. Such a capability would potentially allow an attacker to
create wide-spread panic. In practice, alert systems prevent these
attacks both by controls on sending messages at points where alerts
are originated, as well as by having recipients of alerts verify that
the alert was sent by an authorized source. The former type of
control implemented with local security on hosts from which alerts
can be originated. The latter type implemented by digital signatures
on alert messages (using channel-based or object-based mechanisms).
Alerts typically reach end recipients via a series of intermediaries.
For example, while a national weather service might originate a
hurricane alert, it might first be delivered to a national gateway,
and then to network operators, who broadcast it to end subscribers.
+------------+ +------------+ +------------+
| Originator | | Originator | | Originator |
+------------+ +------------+ +------------+
| . .
+-----------------+..................
|
V
+---------+
| Gateway |
+---------+
|
+------------+------------+
| |
V V
+---------+ +---------+
| Network | | Network |
+---------+ +---------+
| |
+------+-----+ +------+-----+
| | | |
V V V V
+--------+ +--------+ +--------+ +--------+
| Device | | Device | | Device | | Device |
+--------+ +--------+ +--------+ +--------+
Figure 4: Delivering an emergency alert
In order to verify alert signatures, recipients must be provisioned
with the proper public keys for trusted alert authorities. This
trust may be "piece-wise" along the path the alert takes. For
example, the alert relays operated by networks might have a full set
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of ceritificates for all alert originators, while end devices may
only trust their local alert relay. Or devices might require that a
device be signed by an authorized originator and by its local
network's relay.
This scenario creates a need for multiple signatures on alert
documents, so that an alert can bear signatures from any or all of
the entities that processed it along the path. In order to minimize
complexity, these signatures should be "modular", in the sense that a
new signature can be added without a need to alter or recompute
previous signatures.
5. Other Requirements
[[ For the initial version of this document, this section is a
placeholder, to incorporate any further requirements not directly
derived from the above use cases. ]]
6. Acknowledgements
Thanks to Matt Miller for discussions related to XMPP end-to-end
security model, and to Mike Jones for considerations related to
security tokens and XML security.
7. IANA Considerations
This document makes no request of IANA.
8. Security Considerations
The primary focus of this document is the requirements for a JSON-
based secure object format. At the level of general security
considerations for object-based security technologies, the security
considerations for this format are the same as for CMS [RFC5652].
The primary difference between the JOSE format and CMS is that JOSE
is based on JSON, which does not have a canonical representation.
The lack of a canonical form means that it is difficult to determine
whether two JSON objects represent the same information, which could
lead to vulnerabilities in some usages of JOSE.
9. ReferencesBarnes Expires April 25, 2013 [Page 13]